EVOH is a useful polymeric material superior in oxygen barrier properties, flavor retentiveness, oil resistance, antistatic properties and mechanical strength, and has been widely used after molded into films, sheets and vessels. As a process for producing EVOH, it is general that an ethylene-vinyl ester copolymer obtained by copolymerizing ethylene and fatty vinyl ester such as vinyl acetate is saponified in an organic solvent containing alcohol in the presence of a saponification catalyst.
It is described in JP-A-90927/1999 (EP-A-937557) that a solution for strand production is prepared preferably by adding water to an alcoholic solution of EVOH obtained by saponification. It is also described therein that the concentration of EVOH in the EVOH solution thus obtained is preferably 15 to 55 weight % and the alcohol/water weight mixing ratio in the solution is preferably 9/1 to 3/7. A process is described in Example 1 thereof such that a methanol aqueous solution having a water content of 62.5 weight % is fed to a methanol solution of EVOH having a resin concentration of 30 weight % at the azeotropic conditions of a temperature of 100 to 110° C. and a pressure of 3 kg/cm2G to distill methanol up to a resin concentration of 40 weight % and then obtain a fully clear methanol/water uniform solution.
However, when EVOH pellets are produced by a process of precipitating out of the EVOH solution containing large amounts of methanol in a coagulation bath, as described above, alcohol is occasionally volatilized off during the precipitation. The volatilization of alcohol not merely deteriorates the working environment to harm health of the workers but also adversely affects the peripheral environment; therefore, improvements have been sought.
A method for producing an EVOH hydrous composition is described in JP-A-121290/2002 (EP-A-1179546), which method is characterized by introducing an EVOH solution that contains at least 50 parts by weight, relative to 100 parts by weight of EVOH, of an alcohol having a boiling point of not higher than 100° C., into a vessel, and contacting the solution with water vapor in the vessel, thereby letting the alcohol out along with water vapor and then taking an EVOH hydrous composition that contains from 0 to 10 parts by weight of the alcohol and from 10 to 500 parts by weight of water, relative to 100 parts by weight of EVOH, out of the vessel. By this producing method, an EVOH hydrous composition that ensures efficient removal of alcohol not worsening the environment in and aroung working areas can be provided.
It is described in the publication that EVOH hydrous composition pellets are produced by cutting the obtained hydrous composition, and examples of a cutting process include a process of extruding into strands to coagulate and then cutting and a process of directly cutting in melt. It is described therein that the obtained pellets can be produced stably and have a high washing rate and a high drying rate. With regard to the drying, it is described therein that the EVOH pellets are generally dried to lower the water content thereof to at most 1% by weight.
A method of drying EVOH is described in JP-A-81197/2001 (EP-A-1085028), which method is characterized by melting and kneading EVOH with a water content of 5 to 60% by weight until the water content of less than 5% by weight. A process for producing EVOH pellets is described in examples of the publication such that hydrous composition pellets having a water content of 60% by weight, which are obtained by extruding an EVOH solution containing alcohol into a coagulation medium in a strand form for precipitation, thereafter cutting and washsing in water, are subjected to an adjustment of water to a water content of 25 weight % in a dryer and then projected into an extruder for drying up to a water content of 0.15 weight %.
As described in JP-A-90927/1999, however, in the case of washing and drying pellets obtained by cutting a precipitate out of an EVOH solution containing large amounts of methanol in a coagulation bath, there is the possibility that drying time becomes longer, so that not merely production efficiency is reduced but also resin during the drying is deteriorated.
In a process described in JP-A-121290/2002, drying time becomes shorter; however, as described in Example 1 of the publication, it takes 3 hours at a temperature of 100° C. to dry up to a water content of 0.2 weight %, and then further improvements have been desired in view of both production efficiency and the prevention of thermal degradation. In the case of pellets obtained by cutting an EVOH hydrous composition in a molten state, the shape thereof does not necessarily become uniform, and thus there is the possibility that extrusion stability is not sufficiently obtained in molding dried pellet products.
As a method described in JP-A-81197/2001, in the case of drying EVOH hydrous composition pellets having a high water content in an extruder, the total length (L/D) of the extruder is increased so that equipment costs are raised, and additionally thermal degradation is easily caused resulting from kneading time to be extended. On the occasion of melt-kneading an EVOH hydrous composition having a high water content, water is so easily separated from the EVOH that extrusion stability is reduced. Even if water content is previously reduced in a dryer before feeding into an extruder, drying rate in the dryer is not high, so that the problems are easily caused, such as the reduction of production efficiency and thermal degradation.
The present invention has been made to solve the above-mentioned problems and is intended to provide a process for producing EVOH pellets, such as to efficiently remove alcohol without deteriorating the working environment and peripheral environment, to efficiently remove water in the EVOH hydrous composition thus obtained and to obtain less thermal degradation in removing water and pellets having a uniform shape.
Another object of the present invention is to provide an environment-friendly process for producing EVOH resin, such as not to emit carboxylic acid such as acetic acid into the peripheral environment.